Types of Cuttings and Propagation Methods
Types of stem cuttings
It is important that the selected cutting is in good physical condition, free from obvious pests and diseases and with sufficient reserves of food in the form of carbohydrates and proteins to sustain it until it is capable of separate existence and consequent vegetative increase in root, stem and leaf.
It has been stated over the years that soft cuttings are undesirable, this relating to their high ratio of nitrogen to carbohydrates, and while there is undoubtedly some foundation for such a belief, it would not appear to be wholly substantiated by the more modern methods of propagating practised on commercial holdings. Here soft, actively growing cuttings of and other plants are often preferred to those of a harder nature. There is obviously a difference between active growth in good light and lush softness, however, and the latter should generally be avoided as there are unlikely to be sufficient food reserves to sustain the tissue.
Whether, in the light of modern technology, it is desirable to follow older practice and let stock plants intended for provision of cuttings become pot bound and harden their growth in any way by limited nitrogen application and other means is open to debate, it now being thought better to encourage more liberal growth. The age of material selected for cuttings varies, but as a general rule younger tissue contains a higher proportion of meristematic (actively dividing) cells.
There are three main types of stem cuttings — soft, semi-hard and hard, but in greenhouse culture we are more concerned with the soft or semi-hard group.
A great variety ofare raised from soft cuttings because the growth of many greenhouse plants is of a soft nature. Soft cuttings are, however, also taken from a vast number of outside plants herbaceous, alpine and shrub. As the name implies, the cuttings are selected from soft new growth which either arises from the base of the plant in the spring, eg basal cuttings from chrysanthemums, cuttings arising from tubers or from the new growth which frequently develops out of doors after a plant has completed flowering, as with pelargoniums. Plants can often be cut back to induce fresh new growth to develop, this being true of ericas and a great many more.
Removing soft cuttings from the plant involves first of all careful selection of balanced non-etiolated (not drawn or spindly) growth which snaps off cleanly, generally but not always below a node, which is the area from which leaves and buds arise and generally possessed of a wealth of meristematic cells. Many plants can be rooted very simply from internodal cuttings, but gardeners generally prefer to take cuttings below a node as their rooting conditions are not as sophisticated as those of the commercial grower; internodal cuttings could suffer a period of die-back and the risk of disease. Certainly with those plants having a soft or hollow stem, nodal cuttings are more reliable.
Heels can be left on some plants to advantage as this exposes a larger cambium area and is very useful for a lot of soft wooded cuttings removed from shrubs such as deutzia, philadelphus and many more. While non-flowering section cuttings are desirable others can be used, although the flowering stem should be removed to avoid using up valuable reserves of food. The base of cuttings should be trimmed, removing stipules or other superfluous tissue which would merely rot if left.
The size of cuttings varies, but 10-15cm (4-6in) is average. Lower leaves should be removed, not only to reduce the transpiration area, but to avoid rotting. Where a shrub has very large leaves (eg. hydrangea) it is permissible to reduce the leaf area by half or thereabouts, but a reasonable area of active leaf is essential to the rooting process, as a soft shrub deprived of its leaves to the extreme tends to debilitate rather than produce root. Pipings of carnations are simply pulled out of a node and, after removing the lower leaves, inserted in the rooting medium.
Apart from some cacti and succulents, all cuttings should be placed in rooting medium as soon as possible. Where rooting hormones are used, the cuttings should be immersed or dipped in this according to directions. Fungicidal waterings or dustings may be advisable, especially with soft material in very bad weather to avoid ever present disease such as botrytis. The rooting period for soft cuttings can vary from 2-3 days for coleus, up to 3-4 weeks or even longer; the average is 10-21 days. A high success rate is usually possible in mist units, propagating cases or polythene tents. In many cases rooting can be highly successful by putting cuttings in jars of water with a shallow layer of water only and leaving this unchanged, simply adding to it as required.
The provision of a humid atmosphere is a basic essential for the successful rooting of all soft stem cuttings. This is achieved by the use of mist propagation units, propagating cases, plastic domes or white polythene tents or bags, the basic objective of these techniques being to reduce transpiration and enable the cutting to conserve its energies for the vital process of producing new roots. It is advisable to group cuttings, the quicker rooting species being kept apart from those that take longer. Quick-rooting dahlias, chrysanthemums and coleus should obviously not be grouped with conifers and heathers. Stating a precise rooting period for each species is difficult owing to varying conditions, but when possible an indication will be given in the cultural notes. Downy leaved species tend to rot in too close an atmosphere such as that under polythene tents because of moisture collecting on the leaves, resulting in botrytis.
An open, free-draining, warm rooting medium is the prime requisite for maintaining cuttings turgid and in good condition for the quick formation of roots. They can readily be housed in boxes or benches of rooting media with bottom heat provided either bywarming cables or mini-bore or under-bench heating. Many cuttings can be rooted, perhaps less quickly but with great success, under cooler conditions, either in a greenhouse or frame, heated or cold, or in white polythene tunnels.
The qualities of rooting media are fully discussed elsewhere and the importance of studying this subject in depth cannot be over-emphasized. In the most general terms mixtures of half peat and half sand or perlite are most frequently used for rooting a wide range of species. Preparation of the cuttings and insertion distances vary according to species; again, in general most soft cuttings are inserted 2.5-5cm (1-2in) apart.
This term relates to material which has become partly lignified or, in other words, has started to become woody, especially at the base. Propagation by means of semi-hard cuttings relates almost exclusively to the propagation of shrubs from July until September, a wide range of shrubs being propagated during this period with shoots carefully pulled off with a heel, which is trimmed of any surplus thin tissue obviously not containing any cambium cells. Rooting time depends not only on the maturity of the tissue (with many species 2-7 weeks is average), but also on temperature provided.
As the name implies, this mode of propagation involves the use of mature lignified wood and is a technique which has application both out of doors and under glass. It is usual to select well-matured one-year-old growth of various lengths: up to 90cm — 1.2m (3-4ft) long in the case of salix (willow); a more usual length is 20-25cm (8-10in), as for blackcurrants; but cuttings can also be very short, eg. 5-8cm (2-3in) in the case of . A heel of older wood is not only advantageous in some cases, but thought to be essential with many conifers, although once again with modern propagation methods views are changing in this direction.
It is essential that each cutting has at least one live bud near its tip, and when the unripe tips of the shoots are removed care should be taken to leave the bud at the apex of the tip, otherwise die-back to the bud will occur with consequent risk of disease. The base is trimmed below a node in the majority of cases. Buds may be left at the base of the cutting when a “stool” is required, as for blackcurrants; conversely where a “leg” is required, eg. for redcurrants, the lower buds are removed, leaving only buds at the tip.
The etiolation of hardwood cuttings by earthing is thought to be desirable for many fruit trees, although more recent techniques concerned with storage of the normal cuttings in temperature controlled bins have given highly successful results.
‘Eyes’ are hardwood cuttings with a section of stem on both sides of the node, as in, although they can readily be propagated from soft shoots also.
Hardwood cuttings are usually taken in the autumn when they may be inserted individually out of doors or in a cool greenhouse or, as with fruit cuttings, stored in heated bins until spring. It is interesting to note that while evergreen species of shrubs bear leaves, deciduous species do not, and this would lead one to assume that only the leaf-bearing species would respond to a heated propagating case. Such is not always the case, however, and quite dramatic results have been obtained by inserting nearly dormant hardwood cuttings bereft of leaves into warm propagating benches, where under the influence of moisture and heat they have broken into growth and been rooted with ease (eg roses). Research is proceeding with the rooting of a great many other species of dormant hardwood cuttings under propagating house conditions and under white polythene.
These are not commonly used for greenhouse plants, although many species of shrubs and border plants may conveniently be rooted in a frame or greenhouse. The method is to lay suitably sized portions of root (in the case of Phlox paniculata these are about 3.8-5cm (1-½-2in) long in a box of compost, covered by about 1.25-1.9cm (½-3/4in) of the same compost; but here again reference should be made to books relating to the propagation of shrubs, herbaceous plants and fruit trees. In the case of phlox root cuttings are used not only for convenience but to avoid the spread of eel-worm. Root cuttings may sometimes not give rise to identical progeny.
Leaf and leaf petiole cuttings
The production of adventitious buds in the fleshy leaves of many plants is well known, the best example being Begonia rex, where the fleshy leaves, if cut at the main veins and pegged down with small pebbles or rooting media in a propagating case, will invariably produce little plants at the points of incision. Leaf petioles of streptocarpus and saintpaulia are well known examples of greenhouse plants propagated by detaching the plant with the petioles, which are inserted fairly deeply in a rooting medium in pots or boxes. Streptocarpus leaves are often better reduced to half their size to avoid excess moisture loss. Many shrubs also lend themselves to this technique.
Leaf bud cuttings
This involves the use of a bud stem section with the cambium exposed either by removing a bud, as carried out for rose budding, or with a leaf left in the case of camellias, although both of these can be propagated by leaving the stem intact and making a cut at its base. In both cases the cuttings are inserted around the edge of a pot.
Meristem propagation (now termed tissue or cell culture)
This is a technique used by specialist raisers whereby the tip of the growing plant involving the meristem is selected and grown in agar solution. It is thought that this area is free from fungal and virus infection. In addition, the technique of cell culture has increased where portions of the plant, usually the meristem area, are split into cells which are then encouraged to grow in sterile media. This also has considerable implications in respect of disease elimination and ability to produce large numbers of plants from exclusive plant species.
There are now several commercial firms specializing in the production of a wide range of plants using themethod and it is possible to buy young tissue cultured plants for growing on, provided the quantity required is sufficiently large. In actual fact, many gardeners buying a wide range of herbaceous plants, shrubs or pot plants can be sure that a high percentage of these are now produced from , and results in the majority of cases are excellent.
This has not a great deal of application in greenhouse culture, although an exception is chlorophytum; the flower tips can be pegged to the ground. Layering can be applied, however, to a vast range of woody specimens grown in the greenhouse, when it is convenient to layer a stem into a pot of compost. Border carnations grown indoors are also typical of thetechnique, when stems are pegged down into compost after making a short elongated cut through the stem.
Air layering can also be carried out with plants such as ficus which become too tall, although it can also be applied to a number of species by removing a ring around the stem, dusting the wound with a hormone powder before wrapping moist sphagnum moss around the wound and covering this with a ring of polythene secured carefully top and bottom with rubber bands. The polythene allows same air to enter but prevents loss of moisture.